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=Chorismate Mutase=

Introduction
The gene Rv1885c from Mycobacteria tuberculosis encodes for a non-functional chorismate mutase (*MtCM). This non-functional mutase has a 33-amino-acid cleavable sequence. Chorismate mutase is a vital enzyme in the shikimate pathway, which allows for the synthesis of tryptophan, tyrosine, and phenylalanine. This protein acts at the first branch point of the shikimate pathway, making it a regulating step in the conversion of prephenate from chorismate. Since chorismate mutase catalyzes a claisen rearrangement it can be considered an isomerase since it catalyzes rearrangements of isomers. Chorismate mutase provides a 2x106 fold increase in the rate of reaction in comparision to the uncatalyzed reaction.

Chorismate mutase only occurs in bacteria, higher plants, and fungi, due to the fact that the shikimate pathway is only found in these organisms. In Escherichia coli, chorismate mutase has a periplasmic destination. In M. tuberculosis there is in abscence of a periplasmic compartment for chorismate mutase, so it secretes into the culture filtrate of M. tuberculosis. It is believed that a pseudoperiplasmic space might exist in M. tuberculosis.

The N-terminal sequence of M. tuberculosis chorismate mutase is able to function in E. coli which suggests that M. tuberulosis chorismate mutase belongs to the AroQ class of the chorismate mutases.

Rv1885c is synthesized along with the 33-amino-acid terminal sequence, which when expressed with E. coli, is cleaved off the mature protein. Chorismate mutase is the only example of an enzyme catalyzing a percyclic reaction.

Structure


Chorismate mutase is a homodimer which has a predominantly α-helical structure. There are 10 α-helices spread across the two monomers of chorismate mutase. Approximately 86% of the amino acid residues are in the α-helical formations. The α-helical structure of *MtCM are similar to the chorismate mutases of S. cerevisae and E. coli. It holds its dimeric state in a protein concentration as low as 5 nM. There are no β-sheets present in chorismate mutase.

Chorismate has an active site, which is used for the catalysis of the shikimate pathway. The active site is made of Arg 49,Lys 60, Arg 72, Thr 105, Glu 109, and Arg 134. This active site exists through electrostatic interactions with chorismate and hydrogen bonding between the amino acids. The active site forms within a single chain. The active site can form without any help from the second half of the dimer.

The molecular weight of *MtCM is 36,000 Da. Based on that each monomeric subunit has a molecular weight of 18,474 Da, the molecular weight of the molecule supports the theory that it is a dimer. This is also supported by that all chorismate mutases that occur naturally are either trimers or dimers. M. tuberculosis chorismate mutase is similar to the chorismate mutases of yeast and E. coli in the regards that they all are homodimers.

There are no allosteric regulatory sites on *MtCM, which supports the theory that chorismate mutase is not regulated by the aromatic amino acids that are the products of the shikimate pathway.

There is one disulfide bridge in chorismate mutase. It is between Cys 160 and Cys 193.


 * MtCM has a 33-amino-acid cleavable sequence. The N-terminal sequence of M. tuberculosis chorismate mutase is able to function in E. coli which suggests that M. tuberulosis chorismate mutase belongs to the AroQ class of the chorismate mutases.  Another factor that suggests that M. tuberculosis chorismate mutase belongs to the AroQ class of chorismate mutases is that it has has a predominantly α-helical structure, which is similar to the chorismate mutases of E. coli and yeast, which also belong to the AroQ group of chorismate mutases.

Mechanism
in Michaelis-Menten kinetics chorismate mutase has Km of 0.5 ± 0.05 mM and Kcat of 60 s-1.

Chorismate mutase is an essential enzyme in the shikimate pathway. This pathway allows for the biosynthesis of aromatic amino acids tryptophan, tyrosine, and phenylalanine. The production of tyrosine and phenylalanine is achieved by what is called a Claisen rearrangement. First by converting chorismate to prephenate. Prephenate then reacts with prephenate dehydratase and prephenate dehydrogenase which forms phenylpyruvate and hydroxyphenylpyruvate. After this occurs, aminotransferase converts hydroxy-phenylpyruvate and phenylpyruvate to phenylalanine and tyrosine. Chorismate mutase provides a 2x106 fold increase in the rate of reaction, in comparison to the uncatalyzed reaction. It is the only example of an enzyme catalyzing a percyclic reaction.

Chorismate mutase has optimal performance at 37 degrees Celcius and at pH 7.5, but it can still optimally a pH range from pH 4.0 to 7.5

Chorismate Mutase and Tuberculosis
Tuberculosis has developed various mechanisms to survive in hostile environments. The emergence of multi-drug resistant tuberculosis and other diseases such as AIDS compound the problem of how to treat tuberculosis. Chorismate mutase may be involved in pathogenesis. Researchers are currently looking into new antimicrobial drugs for diseases such as tuberculosis. These new drugs would take advantage of the fact that chorismate mutase and the shikimate pathway do not occur in humans, to target and treat various forms of tuberculosis. Chorismate mustase is believed to have a role in the survival of M. tuberculosis. Two genes in M. tuberculosis, Rv0948c and Rv1885c code for chorismate mutase. These help support M. tuberculosis when aromatic amino acids, such as tryptophan, tyrosine, and phenylalanine, are deficient. Some researchers have proposed that a proline-rich section of M. tuberculosis chorismate mutase might be responsible for it binding to the surface receptors on the host cell marcophages .